docs: def header_length(header):
      """Calculates the ciphertext message header length, given a complete header.
  
      :param header: Complete message header object
      :type header: aws_encryption_sdk.structures.MessageHeader
      :rtype: int
      """
      # Because encrypted data key lengths may not be knowable until the ciphertext
      #  is received from the providers, just serialize the header directly.
      header_length = len(serialize_header(header))
      header_length += header.algorithm.iv_len  # Header Authentication IV
      header_length += header.algorithm.auth_len  # Header Authentication Tag
      return header_length